Autoinjector

ABSTRACT

An autoinjector comprising an outer housing in which is mounted a syringe comprising
         a barrel for holding a volume of medicament,   a needle at one end of the barrel and   a plunger axially-moveable in the barrel,
 
the autoinjector further comprising an inner housing intermediate the outer housing and the syringe and an energy source in communication with said inner housing,
   wherein the inner housing is moveable by the energy source between three positions, namely
           a first position in which the inner housing is in communication with the barrel such that, in use, the plunger and barrel are moveable axially so as to move at least part of said needle out of the outer housing;   a second position in which the inner housing is in communication with the plunger but not the barrel such that, in use, said plunger is moveable axially into said barrel so as to expel medicament through the needle; and   a third position in which the inner housing is in communication with neither the plunger nor the barrel such that, in use, the plunger and barrel are able to retract in order to retract the needle into the outer housing,
 
characterised in that the plunger includes biasing means for axially biasing the barrel, before activation of the energy source, to a position forward of the part of the inner housing which acts on the barrel in said first position.

This invention relates to the field of autoinjectors for theadministration of liquid medication.

BACKGROUND

An autoinjector is an automatic injection device designed to facilitatedelivery of a dose of medicament to a patient through a hypodermicneedle, the injection usually being administered by the patientthemselves. An autoinjector works, for example, by delivering aninjection automatically upon actuation by the patient pressing a button,moving a lever or part of a housing etc. This is in contrast to aconventional manual syringe where the patient themselves needs todirectly depress a plunger into a barrel containing medicament in orderto effect the injection. The terms “autoinjector” and “injection device”are used interchangeably in the following description.

An autoinjector is described in our international patent applicationpublished under number WO 2005/070481. Some of the reference numerals inthe present application correspond with the equivalent components in thedevice described in WO 2005/070481. This device requires that the needleis moved axially so that it can appear beyond the end of the nozzle forthe duration of the injection, after which the needle retractsautomatically, so that it is never in sight of the user. The device alsorequires that the plunger is moved axially so that medicament isejected. The overall complexity of the autoinjector is significantlyreduced by both of these requirements being effected by one component,namely an inner housing (illustrated in FIG. 1 of the presentapplication) and the device has the significant advantage that it can bebuilt around a conventional or standard syringe presentation.

The injection device of WO 2005/070481 is designed to be used inconjunction with a standard drug presentation e.g. a syringe comprisinga needle, barrel preloaded with medicament and a plunger. There is asignificant commercial advantage in being able to use a standardsyringe, which will have been subjected to numerous clinical trials,drug stability studies and regulatory approval. Any modification to thestandard syringe may require further trials and approval, adding delayand expense. The present invention is relevant to any injection devicefor use in conjunction with a syringe (whether preloaded or not andwhether single-use or reusable), not only the injection device describedin WO 2005/070481.

In the known device described in patent application no WO 2005/070481,the syringe is supported within the injection device by a barrel orsyringe holder 9. The syringe holder 9 comprises an elongate housingwhich closely surrounds the glass barrel of the syringe. The annularflange 90 at the rear of the syringe barrel rests on a barrel seat 91 atthe rear of the syringe holder 9. The annular flange 90 at the rear ofthe syringe barrel is often referred to as a “finger flange” because,during a conventional (manual) injection using a syringe, the user'sindex and middle fingers rest naturally in front of the “finger flange”in order to provide the necessary resistance to allow depression of theplunger by the thumb to deliver the medicament. The barrel seat 91preferably prevents forward axial movement of the syringe with respectto the syringe holder so that, in use, the syringe barrel and thesyringe holder move axially together as one unit.

The inner housing 7 shown in FIG. 1 includes rear tags 7A which can flexradially into and out of contact with the plunger of the syringe andfront tags 7B which can flex radially into and out of contact with thefinger flange of the syringe barrel.

In use, as described in WO 2005/070481, there are three stages ofdelivering an injection. Before delivering an injection (referring toFIG. 2 of the present application), the end cap 15 is pulled off, takingthe rigid needle cover 17 (if present) and rubber needle sheath with it.In the first stage of delivering an injection, as shown in FIG. 3 of thepresent application, the tags 7B at the forward end of the inner housing7 are in contact with the syringe barrel 90, which is pushed axiallyforward (taking the syringe holder 9 with it), so that the needle 10,which is fixed to the front end of the barrel, moves in the directionindicated by the arrow so that eventually it protrudes beyond the nozzle11 at the front of the device. Forward travel of the barrel and syringeholder is limited when a surface 9A of the syringe holder reaches anendstop 11A inside the nozzle or front housing 11.

Referring now to FIG. 4, the second stage of the injection is thedelivery of the medicament wherein the tags 7A at the rear of the innerhousing 7 depress the plunger 8 into the barrel of the syringe. Duringthis stage, the barrel of the syringe is held axially stationary, byabutment of the annular “finger” flange 90 against the barrel seat 91,which results in the barrel being placed in tension as the plungerpushes the non-compressible liquid medicament towards the forward end ofthe barrel. This tension is undesirable in a glass barrel, which maybecome damaged or broken, especially if the medicament comprises aparticularly viscous liquid which requires greater force to expel itfrom the syringe via the needle. Viscous medicaments are desirable incertain applications, where the use of a sustained-release viscousmedicament reduces the frequency that an injection is required. Theundesirable tension on the barrel during injection can be reduced byusing an alternative syringe holder 100 which is shown in FIG. 5 anddiscussed in our UK patent application number 0620163.6. This type ofsyringe holder supports the barrel at its front end, using for exampleradially-flexible fingers 108 which have gripping means on the interiorsurface thereof.

In the third stage of the injection (not illustrated in the presentapplication but shown in WO 2005/070481), once the medicament has beendelivered and the inner housing 7 is no longer in contact with thebarrel or plunger of the syringe, the secondary spring 12 pushes thesyringe holder (and hence the syringe contained therein) axiallyrearwardly so as to retract the syringe back into the housing so thatthe used needle is concealed from view.

FIG. 6 shows the device from GB0620163.6, in its storage condition. Notethat the flange 90 of the barrel is forward of the front tags 7B of theinner housing 7. Whilst the endcap 15 is in place, none of the internalcomponents of the device can move axially relative to one another.

When the endcap 15 and needle sheath 17 are removed so that the deviceis ready for use, it is possible that the syringe will have a tendencyto move slightly axially with respect to the outer housing and the innerhousing 7. This undesired axial movement could be as a result of“bounce-back” caused by the sudden release of the needle sheath from theneedle, or could depend upon the orientation in which the user isholding the device, for example.

This undesired axial movement of the syringe could be prevented bystoring the tags 7B in a radially-inward position, i.e. already abuttingflange 90 so that it is impossible for the syringe to move rearwardlywith respect to the inner housing 7. However, this solution isundesirable because, after storage for a prolonged period of time, it ispossible that the plastic tags 7B will acquire “memory” of theirradially-inward position such that, during operation of the device, theyare not capable of springing radially-outwardly when required to be outof engagement with the syringe barrel (i.e. during the third stagedescribed above).

The performance of the radially-flexible tags 7B during delivery of aninjection is optimised by storing them in a radially-outward position(as illustrated in FIG. 6) so that they can be temporarily forcedinwardly into contact with the flange 90 when required to move thesyringe and its attached needle forward for delivery of an injection,after which they can easily spring back into the radially-outwardposition (of which they have acquired “memory”) after injection when itis desired for the syringe to retract fully into the housing. However,storage of the tags 7B in a radially-outward position means that thereis nothing to prevent the undesired axial movement mentioned above whenthe endcap 15 and needle sheath 17 are removed.

If the undesired axial movement is rearward such that the flange 90 ofthe barrel comes to rest in a position rearward of the tags 7B, it willnot be possible for the device to operate properly because the tags 7Bwill be unable to engage with the flange in order to move the syringeaxially forward as is necessary during the first stage of the injectiondescribed above. The failure of the device to operate properly wouldonly become apparent upon actuation of the injection causing uncertaintyfor the patient and risk to health resulting from incomplete or no doseof medicament being delivered.

A second disadvantage of the prior art arises due to variations in theinitial absolute axial position of the rear flange of the plunger fromone syringe to the next. The rear flange of the plunger needs to beengaged by the rear tags of the inner housing in order to deliver aninjection. These variations in initial axial position could, forexample, be caused by manufacturing tolerances, variations in thefilling of the syringe with medicament, variations in the volume of thegas bubble inside the syringe, changes in atmospheric pressure (forexample if the device is transported by airfreight) etc. These slightvariations may affect the volume of the dose of medicament beingdelivered. The dose of medicament to be delivered is determined by theextent of the axial movement of the plunger into the syringe barrel,which in turn is controlled by the relative positioning of the tags 7A,7B on the inner housing 7. For a given axial distance travelled by theinner housing (and the rear tags thereof), the actual dose of medicamentdelivered may vary slightly depending upon the absolute axial startingposition of the plunger. This problem is particularly relevant indevices which are designed to deliver a partial dose (e.g. half orquarter) of a full syringe, compared with devices which are designed todeliver a full dose (i.e. completely emptying the syringe). The absoluteaxial starting position of the plunger may therefore be of importance inmaintaining the accuracy of the actual dose of medicament delivered.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is providedan autoinjector comprising an outer housing in which is mounted asyringe comprising

-   -   a barrel for holding a volume of medicament,    -   a needle at one end of the barrel and    -   a plunger axially-moveable in the barrel,        the autoinjector further comprising an inner housing        intermediate the outer housing and the syringe and an energy        source in communication with said inner housing,    -   wherein the inner housing is moveable by the energy source        between three positions, namely        -   a first position in which the inner housing is in            communication with the barrel such that, in use, the plunger            and barrel are moveable axially so as to move at least part            of said needle out of the outer housing;        -   a second position in which the inner housing is in            communication with the plunger but not the barrel such that,            in use, said plunger is moveable axially into said barrel so            as to expel medicament through the needle; and        -   a third position in which the inner housing is in            communication with neither the plunger nor the barrel such            that, in use, the plunger and barrel are able to retract in            order to retract the needle into the outer housing,            characterised in that the autoinjector includes biasing            means for axially biasing the barrel, before activation of            the energy source, to a position forward of the part of the            inner housing which acts on the barrel in said first            position.

Preferably, the biasing means acts on said plunger of the syringe andthe incompressible nature of the liquid medicament in the syringetransmits the biasing force to the barrel.

In one embodiment, said biasing means comprises a compression springheld between the outer housing and the plunger. Preferably saidcompression spring is integrally-formed with said plunger. In apreferred form, said compression spring is provided with a rear flangefor abutment with a part of said outer housing.

In another embodiment, the biasing means is associated with the frontend of said inner housing, located in the axial path of the barrel.Usually, the biasing means is not strong enough to resist a forwardaxial force provided by said energy source in communication with saidinner housing. The biasing means may be one or more radially-flexiblelegs integrally-formed with said inner housing.

In an alternative embodiment, said biasing means comprises a chambercontaining a piston in a shear thickening fluid which, upon activationof said energy source, is capable of transmitting forward axial forcefrom said energy source to the plunger and a compression spring heldbetween the outer housing and said piston.

According to a second aspect of the present invention, there is providedan autoinjector comprising an outer housing in which is mounted asyringe comprising

-   -   a barrel for holding a volume of medicament,    -   a needle at one end of the barrel and    -   a plunger axially-moveable in the barrel,        the autoinjector further comprising an inner housing        intermediate the outer housing and the syringe and an energy        source in communication with said inner housing,    -   wherein the inner housing is moveable by the energy source        between three positions, namely        -   a first position in which the inner housing is in            communication with the barrel such that, in use, the plunger            and barrel are moveable axially so as to move at least part            of said needle out of the outer housing;        -   a second position in which the inner housing is in            communication with the plunger but not the barrel such that,            in use, said plunger is moveable axially into said barrel so            as to expel medicament through the needle; and        -   a third position in which the inner housing is in            communication with neither the plunger nor the barrel such            that, in use, the plunger and barrel are able to retract in            order to retract the needle into the outer housing,            characterised in that the autoinjector further comprises            means associated with a rear flange of said plunger            providing a plurality of axial positions at which it is            possible for said inner housing to engage said plunger in            said second position.

The means providing a plurality of axial positions is associated withthe rear flange of the plunger because it is the rear flange of theplunger which is “picked up” by the inner housing in the second positionto depress the plunger into the syringe barrel.

In one embodiment, the means associated with said rear flange of theplunger comprises a chamber containing a piston in a shear thickeningfluid which, upon activation of said energy source, is capable oftransmitting forward axial force from said energy source to the plungerand a compression spring held between the outer housing and said piston.

In an alternative embodiment, the means associated with said rear flangeof the plunger comprises a ratchet cap providing an axial range ofengagement points for said inner housing in said second position.

Preferably, the autoinjector is a single-use autoinjector. The simpleconstruction of the autoinjector makes it very appropriate forapplications such as emergency use for injecting a large population tocontrol a pandemic, where a large number of cost-effective disposableautoinjectors are required. A single-use autoinjector also provides avery convenient means for patients to administer their own injections,even if lacking in dexterity and/or clinical experience.

Typically, the energy source, for example a coiled spring, is capable ofmoving said plunger axially in the barrel to deliver an injection inless than 30 seconds.

Preferably, the syringe is axially moveable in said housing and isbiased so that the needle is normally wholly inside said housing,wherein before injection the syringe is movable axially so as to move atleast a part of said needle out of the housing and wherein afterinjection, the syringe is able to retract in order to retract said partof said needle into the housing. The concealment of the needle bothbefore and after injection makes the autoinjector particularly suitablewhere the patient has any aversion to injection by needle. Concealmentof the needle both before and after injection also eliminates the riskof needle-stick injury.

Further features of the invention are defined in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be moreparticularly described, by way of example only, with reference to theaccompanying drawings in which:

FIG. 1 (PRIOR ART) is a perspective view of the inner housing from WO2005/070481;

FIG. 2 (PRIOR ART) is a perspective view of a known injection device;

FIG. 3 (PRIOR ART) is a plan view, partly in section of the FIG. 2device, with the cap and needle cover removed, ready for actuation;

FIG. 4 (PRIOR ART) is a plan view, partly in section of the FIG. 2device, with the needle exposed, ready for the plunger to be depressedin order to deliver the medicament;

FIG. 5 (PRIOR ART) is a perspective view of an alternative syringeholder from GB0620163.6;

FIG. 6 (PRIOR ART) is a perspective view partly in section showing theinner housing in relation to the syringe holder from GB0620163.6;

FIG. 7 is a perspective view partly in section showing the inner housingin relation to the syringe holder, the syringe having a modified plungeraccording to a first embodiment of the invention;

FIG. 8 is a perspective view of a modified inner housing;

FIG. 9 is a perspective view of a further modified inner housing,embodying a second aspect of the invention;

FIG. 10, drawn to a larger scale, shows further detail of a part of theinner housing of FIG. 9;

FIG. 11 is a cross-sectional view of part of the inner housing of FIG.9;

FIGS. 12-17 are schematic cross-sectional views of a second embodimentof the invention, showing various stages of operation of the device;

FIG. 18 is a schematic cross-sectional view of the rear part of theplunger having a ratchet cap; and

FIG. 19 is a schematic cross-sectional view of the rear part of theplunger having an alternative embodiment of the ratchet cap.

DETAILED DESCRIPTION

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, means “including but not limited to”, andis not intended to (and does not) exclude other components, integers orsteps.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Throughout the following description, reference to a “forward” directionmeans the direction which is towards the patient when the injectiondevice is in use. The “forward” end of the injection device is the endnearest the patient's skin when the device is in use. Similarly,reference to a “rearward” direction means the direction which is awayfrom the patient and the “rearward” end of the device is the endfurthest from the patient's skin when the injection device is in use.

The “plunger” does not necessarily (but may) include any elastomericstopper or the like which seals the chamber containing liquidmedicament.

Features, integers, characteristics or groups described in conjunctionwith a particular aspect, embodiment or example of the invention are tobe understood to be applicable to any other aspect, embodiment orexample described herein unless incompatible therewith.

Referring to FIG. 7, a first embodiment of the invention is illustrated.In this embodiment, there is a biasing means in the form of acompression spring 200 between the rear of the outer housing 30 and theflange 8F of the plunger 8. The illustrated spring 200 is integrallyformed with the plunger flange 8F, but alternatively, a separate springcould be provided. The spring 200 has a flange 201 at the rear thereofto abut the rear of the outer housing. The spring 200 is a compressionspring so that the plunger flange 8F is biased axially forwards so thatthe syringe as a whole is also biased axially forwards (the biasingforce from the spring 200 being transmitted to the rest of the syringeincluding the barrel via the incompressible liquid medicament therein).This means that the flange 90 of the syringe barrel is always initiallylocated forward of the front tags 7B of the inner housing. The forwardaxial force provided by the spring 200 is relatively weak and needs onlybe sufficient to bias the syringe forwards. The force is not sufficientto cause medicament to be ejected from the front of the device, nor isit sufficient to affect the operation of the main energy source (usuallya much more powerful spring) which actuates the device when delivery ofan injection is required.

Using this modified plunger, the performance of the device is improved,as one always knows that the syringe flange's initial axial position isbiased forward of the rear tags 7B, avoiding the risk of malfunction.

Using the modified plunger, the axial position of the plunger is suchthat it is possible to for the tags 7A, 7B to be stored in theirradially-outward position, which is preferable given the above-describedtendency for the tags to acquire “memory” of their stored position. Inorder to avoid the risk of one set of the tags catching in the recessesintended for the other set of tags, the front tags 7B are radiallyoffset from the rear tags 7A, for example by 45°, as illustrated in FIG.8.

An alternative embodiment of the invention is shown in FIGS. 9-11. Inthis embodiment, the biasing means are provided as part of a modifiedinner housing. Referring to FIG. 9, the inner housing 7 has rear tags 7Aand front tags 7B having the same function as described above.

In addition, and as shown best in FIG. 10, the front end of the innerhousing is provided with one or more biasing means in the form ofradially flexible legs 250. The legs are preferably integrally formedwith the inner housing 7. Each flexible leg 250 may have an enlargedhead 251.

Normally, before the autoinjector is fired, the legs 250 are flexedradially inwards so that they are in the axial path of the finger flange90 of the syringe barrel. FIGS. 10 and 11 show the finger flange 90 ofthe syringe barrel and the barrel seat 91 at the rear of the device'ssyringe holder. The legs 250 may be moulded so that this is theirdefault position. The legs 250 need not abut the finger flange 90(although they may do so) but they must at least partially block itsrearward axial path. In this way, undesirable rearward axial movement ofthe finger flange 90 is prevented and the syringe barrel is biased to aposition forward of the front tags 7B. Again, the performance of thedevice is improved, as one always knows that the syringe flange'sinitial axial position is biased forward of the rear tags 7B, avoidingthe risk of malfunction.

The legs 250 of the inner housing 7 are relatively weak and need only tobe strong enough to resist the weight of the syringe to preventundesirable axial movement. They do not impede the firing of theautoinjector to deliver medicament. When the autoinjector is actuated,the inner housing 7 is urged forward by the full force of the mainenergy source. The legs 250 cannot resist this force and so they areforced radially-outwardly, out of the path of the finger flange 90.Therefore it is possible for the front tags 7B to engage the fingerflange 90 in the normal way to move the syringe axially forward enoughto move the needle out of the housing into the injection site.

An alternative embodiment of the invention is shown schematically inFIGS. 12-17. In this embodiment, the rear of the plunger 8 is attachedto a chamber 300 filled with a shear thickening fluid. A shearthickening fluid is one whose viscosity increases with the rate of shearand is sometimes referred to as a “dilatant” material. The chamber 300is fixed with respect to the plunger 8.

The chamber 300 contains an axially-moveable piston 301 which does notfill the cross-section of the chamber, so that the shear thickeningfluid is able to pass freely around the piston. The piston 301 isattached to and fixed with respect to a rear flange 302. The rear tags7A of the inner housing engage behind the rear flange 302, in the sameway as if the rear flange were the flange of the plunger (8F) in theFIG. 7 embodiment. A compression spring 303 is disposed between the rearflange 302 and the rear surface of the chamber 300, as shown in FIG. 12,so that the rear flange 302 is always biased against the rear tags 7A ofthe inner housing. The outer housing of the device is labelled as item30 and the barrel of the syringe is labelled as item 9 in FIG. 12.

The chamber 300 acts as a linear dashpot and, in combination with thespring 303 may be referred to as biasing means.

FIG. 13 shows what happens when the plunger 8 moves during storageand/or transportation of the device. For example, the pressure insidethe syringe may change if the device is transported by airfreight whenany gas bubble inside the syringe may change in volume, causing theplunger 8 to move axially. The axial movement is relatively slow and isnot forceful enough to change the viscosity of the shear thickeningfluid. Consequently, as the plunger 8 moves rearwardly (upwards asillustrated in FIG. 13) so does the chamber 300. However, the piston 301does not move rearwardly (as it is attached to the rear flange 302) andso the shear thickening fluid moves around it as the chamber movesrearwardly. The distance between the rear flange 302 and the rearsurface of the chamber 300 reduces as the spring 303 is compressed. Ifthe plunger moves forwards, the opposite happens i.e. the distancebetween the rear flange 302 and the rear surface of the chamber 300increases. The spring 303 always keeps the rear flange 302 biasedagainst the tags 7A of the inner housing.

FIG. 14 shows what happens when the device is actuated to deliver aninjection. As the inner housing 7 moves forwards with the rear tags 7Aengaged behind the rear flange 302, the strong driving force of thedevice's main energy source causes the viscosity of the shear thickeningfluid to change and the fluid thickens. The thick fluid can no longermove freely around the piston 301 and the chamber 300 acts, effectively,as a solid bridge between the rear flange 302 and the plunger 8, so thatthe plunger 8 is driven into the barrel 9 of the syringe to deliver themedicament.

FIG. 15 shows what happens when the dose of medicament has beendelivered. When the plunger 8 reaches the end of its forward travelinside the syringe, the piston 301 continues to move forwards, albeit ata reduced speed because the thickened shear thickening fluid is stillable to flow to some extent. Therefore the inner housing 7 is able tocontinue forwards slightly after the full dose of medicament has beendelivered. This extra forward movement ensures that the inner housing 7reaches its desired forward position regardless of its absolute initialaxial starting position. This is necessary in order for the rear tags 7Ato move radially-outwardly releasing the rear flange 302 as shown inFIG. 16.

Thus, although the rear flange 302 is always initially biased againstthe rear tags 7A, the shear thickening fluid chamber provides an axialrange of possible initial starting points for the plunger 8.

Once the rear tags 7A are fully disengaged from the rear flange, asshown in FIG. 16, the rear flange 302, chamber 300 and plunger 8 arefree to move axially rearwardly under the force of a secondary spring atthe front of the device (not illustrated). As shown in FIG. 17, theneedle 10, syringe barrel 9, plunger 8, chamber 300 and rear flange 302can all retract into the device so that the needle is fully concealedfrom the patient's view.

A significant advantage of the chamber of shear thickening fluid is thata precise absolute axial starting point of the plunger is no longerrequired for proper functioning of the device. The chamber of shearthickening fluid absorbs the effect of minor axial movements of theplunger during storage and transportation. Thus, although the rearflange 302 is always initially biased against the rear tags 7A, theshear thickening fluid chamber provides an axial range of possibleinitial starting points for the plunger 8. Regardless of the absoluteinitial starting point for the plunger 8, the shear thickening fluidenables a defined relative axial movement to occur in order to deliverthe required dose.

A further improvement is illustrated in FIG. 18. Here, instead of thechamber of shear thickening fluid, a ratchet cap 400 is provided whichsites over the rear flange 8F of the plunger 8, locked into place bylocking tabs 401. The rear tags 7A of the inner housing can engageanywhere along the ratchet cap 400, depending upon the absolute axialposition of the plunger (and consequently the ratchet cap attachedthereto). This also effectively provides a plurality of axial startingpositions for the plunger for which the device will work to deliver therequired dose of medicament. The ratchet cap embodiment could be used incombination with the modified plunger rod of FIG. 7. Other alternatives,instead of the illustrated ratchet cap, may be used to provide the rangeof axial starting positions for the plunger, for example a differentlyshaped component intermediate the inner housing tags and plunger.Alternatively, the ratchet cap or equivalent component could be anintegral part of a modified plunger, for example a plunger having aplurality of annular ribs.

The diameter of the inner housing 7 may need to be increased slightly toaccommodate the extra diameter of the ratchet cap. Alternatively, theratchet cap could be shaped as shown in FIG. 19 so that the diameter ofthe device does not need to be increased. This embodiment has theadvantage that the overall length of the device could be reduced withthe rear flange positioned further back in the device, inside the mainspring (the device's main energy source).

The reader's attention is directed to all papers and documents which arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings), may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of any foregoingembodiments. The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

1. An autoinjector comprising an outer housing in which is mounted asyringe comprising a barrel for holding a volume of medicament, a needleat one end of the barrel and a plunger axially-moveable in the barrel,the autoinjector further comprising an inner housing intermediate theouter housing and the syringe and an energy source in communication withsaid inner housing, wherein the inner housing is moveable by the energysource between three positions, namely a first position in which theinner housing is in communication with the barrel such that, in use, theplunger and barrel are moveable axially so as to move at least part ofsaid needle out of the outer housing; a second position in which theinner housing is in communication with the plunger but not the barrelsuch that, in use, said plunger is moveable axially into said barrel soas to expel medicament through the needle; and a third position in whichthe inner housing is in communication with neither the plunger nor thebarrel such that, in use, the plunger and barrel are able to retract inorder to retract the needle into the outer housing, characterised inthat the autoinjector includes biasing means for axially biasing thebarrel, before activation of the energy source, to a position forward ofthe part of the inner housing which acts on the barrel in said firstposition.
 2. The autoinjector of claim 1 wherein said biasing means actson said plunger.
 3. The autoinjector of claim 1 wherein said biasingmeans comprises a compression spring held between the outer housing andthe plunger.
 4. The autoinjector of claim 3 wherein said compressionspring is integrally-formed with said plunger.
 5. The autoinjector ofclaim 3 wherein said compression spring is integrally-formed with saidouter housing.
 6. The autoinjector of claim 3 wherein said compressionspring is separable from said plunger and/or said outer housing.
 7. Theautoinjector of claim 3 wherein said compression spring is provided witha rear flange for abutment with a part of said outer housing.
 8. Theautoinjector of claim 1 wherein said biasing means is associated withthe front end of said inner housing.
 9. The autoinjector of claim 8wherein said biasing means is located in the axial path of said barrel.10. The autoinjector of claim 9 wherein said biasing means is not strongenough to resist a forward axial force provided by said energy source incommunication with said inner housing.
 11. The autoinjector of claim 8wherein said biasing means comprises a radially-flexible legintegrally-formed with said inner housing.
 12. The autoinjector of claim11 wherein said inner housing is provided with a plurality of saidradially-flexible legs.
 13. The autoinjector of claim 1 wherein saidbiasing means comprises a chamber containing a piston in a shearthickening fluid which, upon activation of said energy source, iscapable of transmitting forward axial force from said energy source tothe plunger, and a compression spring held between the outer housing andsaid piston.
 14. The autoinjector of claim 1 further comprising meansassociated with a rear flange of said plunger providing a plurality ofaxial positions at which it is possible for said inner housing to engagesaid plunger in said second position.
 15. The autoinjector of claim 14wherein said means comprises a chamber containing a piston in a shearthickening fluid which, upon activation of said energy source, iscapable of transmitting forward axial force from said energy source tothe plunger, and a compression spring held between the outer housing andsaid piston.
 16. The autoinjector of claim 14 wherein said meanscomprises a ratchet cap intermediate said inner housing and saidplunger. 17-37. (canceled)